Democracy Forward, a liberal-leaning legal organization that frequently battled the first Trump administration in court, on Thursday unveiled a large-scale new effort aimed at thwarting President-elect Donald J. Trump’s second-term agenda from his first day in office.

More than 800 lawyers at 280 organizations have begun developing cases and workshopping specific challenges to what the group has identified as 600 “priority legal threats” — potential regulations, laws and other administrative actions that could require a legal response, its leaders said. The project, called Democracy 2025, aims to be a hub of opposition to the new Trump administration.

Unlike in 2017, when Democratic lawyers were unprepared for the onslaught of conservative policies, the intent is to be ready to unleash a flurry of lawsuits immediately.

“We’re leveling up and lawyering up,” Skye Perryman, the chief executive of the organization, said. “This wasn’t something that just everybody woke up the day after the election and started to plan.”

Democracy Forward has spent the last two years working to identify the possible actions the new Trump administration could take on issues Democrats see as key priorities to defend, the group’s leaders said, using as a blueprint Mr. Trump’s first-term actions, his campaign promises and plans released by his allies, including the Heritage Foundation and its Project 2025 agenda.

Those issues include abortion rights, health care, climate, union protections, environmental protections and immigration. But the group has also given particular weight to Mr. Trump’s promises to weaponize the systems of government, particularly the Justice Department, against those he sees as foes along with his vows to dismantle federal agencies and fire thousands of government workers.